Duckweed Propagation Guide

How duckweed (Lemna minor) multiplies by vegetative budding and division, why it doubles in days, and how to control this fast floating plant before it overruns the tank.

Overview

Duckweed, Lemna minor, is the smallest flowering plant in the world and forms a bright green film across still water surfaces. Each plant is a tiny leaf-like frond with a single root, and small air spaces inside the frond keep it afloat. It is an excellent nutrient absorber, which makes it useful for soaking up excess nitrogen and phosphorus.

Under good conditions it grows extremely fast — productive enough to yield up to 73 tonnes of dry matter per hectare a year — so the gardener's task is rarely to encourage it and almost always to keep it in check.

Propagation Method

Duckweed reproduces mainly vegetatively by budding and division. As a frond grows it buds daughter fronds along its margin, and as those daughters mature the plants split into separate individuals, each carrying its own single root. Flowers are produced only rarely — about 1 mm across, with 1 mm seeds bearing 8–15 ribs — so sexual reproduction is uncommon and propagation in the aquarium is effectively automatic.

Step-by-Step

1. Add a small handful of healthy fronds to a calm, well-lit water surface.
2. Leave the surface free of strong current so fronds stay together and bud freely.
3. Let each frond bud daughter fronds; the colony divides on its own within days.
4. Scoop out a portion of the floating mat to start a new culture elsewhere.
5. Keep the donor tank thinned so light still reaches the plants below.

Conditions for Healthy Growth

• pH between roughly 6.5 and 8 is ideal, with growth viable from about 6 to 33 °C.
• Calm surface water with little or no flow.
• Nutrient-rich water — duckweed thrives on excess nitrogen and phosphorus.
• Bright light at the surface to drive its rapid multiplication.

Maintenance

Skim out excess duckweed every couple of days with a fine net to keep a thin layer rather than a thick blanket. Regular removal prevents the film from sealing off the surface and is also the simplest way to export absorbed nutrients from the system.

Common Challenges

The defining challenge is runaway growth: in temperate conditions below about 6–7 °C the plant forms starch-filled turions and sinks dormant for winter, then resurfaces and resumes growth in spring, so it persists across seasons. Birds spread it between ponds because the sticky root clings to feathers and feet. Once established it is very difficult to eradicate, so commit to it only if you are prepared to manage it indefinitely.